The invention relates to fluid power motors and particularly to motors of the type having an encompassing guard tube to protect the cylindrical body from external damage while axially fixing the end caps relative to each other. Such motors are disclosed in Bimba U.S. Pat. No. 4,073,219, the disclosure of which is incorporated by reference herein.
Fluid power cylinders convert fluid power energy into push-pull energy. They must be operated with either compressed air or liquid, and are generally known as pneumatic or hydraulic. Briefly, conventional fluid power cylinders comprise a pressure retaining tubular body section with a smooth internal wall, end cap closures, a piston with grooves and seal rings, and a piston rod which must extend through at least one end cap member of the unit. Whereas a conventional fluid power cylinder has only one tubular section, the guard type cylinder disclosed in Pat. 4,073,219 has two tubular body sections. The outer or guard tube is spaced apart from the inner pressure retaining tube and holds the end cap closures together. The space permits the outer tube, which can be made a relatively thick, inexpensive material such as aluminum, to absorb dents without affecting the thin and expensive stainless steel body cylinder. The maximum distance a piston can travel during its operation is called the stroke length and is determined by the distance between the inner faces of the end caps against which the piston abuts at the end of its travel.
The method of attachment of end caps to the outer guard tube by means of having the end portions of the guard tube internally threaded to engage with the external threading on the end caps, as disclosed in my U.S. Pat. No. 4,073,219, is both a troublesome and expensive aspect of manufacturing a guard tupe fluid power cylinder. The greatest difficulty is in obtaining the desired stroke length within acceptable tolerances. In this type of assembly, after the internal threads of the guard tube make a threaded engagement with the external threads of the ends caps, the end cap must be threaded into the guard tube until an access opening for a pipe connection is brought into alignment with a tapped opening in the end cap. In production threading there is always a variation as to where the first thread will start in relation to a reference point. Thus, in the guard tube, the first thread could start anywhere within 360.degree. of the centerline of the access opening. The same is true with the end cap and its tapped hole. Since there are two end caps, the variation could be great. For a 20 pitch thread, the variation could be 0.050" per each end or 0.100" which would not be acceptable since it is often necessary to control the stroke length to about plus or minus 0.015".